The present invention relates to an adjuster strut for use in a drum brake assembly and to a drum brake assembly that employs an adjuster strut.
Adjuster struts are commonly employed in brake assemblies to facilitate compensation for wear over time in brake friction lining. The adjuster strut typically extends in engagement between opposed braking members, being opposed brake shoes, or a brake shoe and a parking brake lever, and is operative to automatically extend lengthwise upon wear of the friction lining, to maintain a substantially constant gap between the lining and the drum braking surface in a xe2x80x9cbrake offxe2x80x9d or non-braking condition.
Most known adjuster struts generally have bifurcated or forked ends suitable to accept at one end thereof the leading edge of the inwardly depending web of a T-shaped brake shoe, and at the other end thereof, both the leading edge of the inwardly depending web of an opposed, T-shaped brake shoe and the similar edge portion of the parking brake lever positioned adjacent thereto. At least one portion of such a strut is normally formed from tubular metal and the bifurcated end of the strut is typically formed by pressing the end of the tubular portion together and removing a central portion therefrom to form a lengthwise slot. That slot is generally of a width approximately equal to, but slightly greater than the thickness of the sections or portions of the braking member or members which it is to accept. It is also normal for both the webs and the parking brake lever to be formed with slot openings at the desired strut engagement position to accept and support the ends of the strut.
One drawback of the above known strut arrangement is that the material removed from the pressed tubular end portion is wasted and the mechanism of removal, normally by grinding or stamping, requires a specific manufacturing step. Moreover, such struts can be awkward to install, given that the bifurcated ends are required to straddle the respective web and lever edges.
A further drawback of known adjuster struts is the manner in which the ends thereof are connected or supported in the brake assembly. In some prior art arrangements, the ends of the adjuster strut have a significant lengthwise bifurcation to such an extent to ensure that the adjuster strut remains engaged at either end with the braking members on radial expansion of the brake shoes, even if the strut malfunctions so as not to extend itself during that expansion. This arrangement prevents release of one or both ends of the adjuster strut from engagement with the relevant braking members, as such release is undesirable, by leaving the strut unsecured within the rotating wheel cavity and permitting the strut to possibly damage the components in that cavity or jam the brake assembly against operation, with obvious and potentially catastrophic consequences.
An alternative prior art arrangement adopted in this respect employs biasing springs to permanently secure the ends of the adjuster strut in engagement with the braking members. Such adjuster struts generally employ an adjuster mechanism that includes a nut threaded on a shaft and rotation of the nut by the adjuster mechanism permanently shifts the axial position of the shaft to lengthen the adjuster strut. The major length of the shaft is accommodated within an unthreaded tube so that the shaft can rotate freely within the tube and shift axially without obstruction. In the case of malfunction/seizure of the adjuster mechanism, expanding movement of the brake shoes can be accommodated as necessary through lengthwise expansion of the adjuster strut, by sliding movement of the shaft within the tube while the ends of the strut are secured to the braking members by the springs. This arrangement therefore secures the strut against release from the braking members, but requires additional spring components and can be awkward to fit.
It is an object of the present invention to provide an adjuster strut that overcomes or at least alleviates one or more of the above drawbacks.
According to the invention there is provided an adjuster strut for use in a drum brake assembly, said adjuster strut having an adjustable elongate extent with first and second engagement means (units) disposed at opposite ends thereof for engaging opposed braking members of a brake assembly, said first engagement means being arranged for receipt and support within an inwardly opening slot formed in one of said braking members and including an end face for abutting engagement with the inner end wall of said slot opening, said first engagement means further including a first channel extending rearwardly from said face, for receiving therein an edge or wall portion (hereinafter an xe2x80x9cedge portionxe2x80x9d) defined by said slot opening.
An adjuster strut of the above kind is distinguished from the prior art by the absence of the need for each end of the strut to be bifurcated. As such, there are reduced levels of wasted material in the strut construction. In particular, the strut can have reduced axial length, because at least one of the ends of the strut does not have to accept the radially inward extending edge of one or both of the braking members. Additionally, the channel formed in the adjuster strut of the invention can be formed as part of the pressing step normally already undertaken in the formation of one of the ends of known struts. Thus, an adjuster strut according to the invention can have less manufacturing steps compared to known struts.
An adjuster strut according to the present invention preferably includes a main portion which is formed from tubular metal section and the end portion for engagement within the slot opening of the braking member is preferably pressed to flatten the end and to form the channel between upstanding side regions to accept the edge portion of the slot opening. Receipt of the edge portion within the channel is such as to locate the respective end of the adjuster strut in at least one plane relative to the braking member to which it is engaged. A second channel may be formed on an opposite side of the channel described above so that the end has a cross-section which his generally dumb-bell shaped, and that second channel is provided to accept a second and oppositely disposed edge portion of the slot opening so that the adjuster strut is engaged by edges of the slot opening on opposite sides thereof.
A slot opening is provided in at least one of the braking members for receipt and support of one end of the adjuster strut, although in a preferred arrangement, each braking member includes a slot opening so that each end of the strut is received and supported in such a slot. In that arrangement, the end face of the or each strut end is preferably arranged for abutting engagement with the inner end of a slot opening at least in the brake off condition of the brake assembly, while the upper and/or the lower edge portions of that opening is/are received in a channel(s) extending rearwardly from the end face.
The invention further provides a drum brake assembly including a rotatable drum, a support, a pair of cross-sectionally T-shaped brake shoes defining a radially inwardly depending web mounted on said support in opposed relationship for substantially radial movement into and out of engagement with a braking surface of said rotatable drum, actuating means facilitating movement of said brake shoes, and an adjuster strut, said adjuster strut having an adjustable elongate extent with first and second engagement means disposed at opposite ends thereof for respectively engaging said brake shoes, said first engagement means being received and supported within an inwardly opening slot formed in the web of a first of said brake shoes and including an end face for abutting engagement at least in a radially contracted or brake off condition of said brake shoes with the inner end wall of said slot opening, said first engagement means further including a first channel extending rearwardly from said face and said slot being arranged so that an edge or wall portion (hereinafter an xe2x80x9cedge portionxe2x80x9d) of said slot opening extends into said channel.
The invention still further provides a drum brake assembly including a rotatable drum, a support, a pair of cross-sectionally T-shaped brake shoes defining a radially inwardly depending web mounted on said support in opposed relationship for substantially radial movement into and out of engagement with a braking surface of said rotatable drum, a parking brake lever disposed adjacent the web of one of said brake shoes, actuating means facilitating movement of said brake shoes, and an adjuster strut having an adjustable elongate extent with first and second engagement means disposed at opposite ends thereof, said first engagement means engaging said brake shoe and said adjacent parking brake lever, and said second engagement means engaging the other said brake shoe, said first engagement means being received and supported within adjacent inwardly opening slots formed in each of the web of the first of said brake shoes and said parking brake lever, and including an end face for abutting engagement at least in a radially contracted or brake off condition of said brake shoes with the inner end wall of at least one of said adjacent slots, said first engagement means further including a first channel extending rearwardly from said face and an edge or wall portion (hereinafter an xe2x80x9cedge portionxe2x80x9d) of at least one of said adjacent slots extending into said channel.
In a brake assembly in which a slot opening is formed in both the brake shoe web and the adjacent parking brake lever, the respective openings may be of different lengthwise extent, preferably with the slot opening of the brake shoe web having greater lengthwise extent than that of the parking brake lever. This arrangement is such that the relevant end face of the strut engages only the inner end of the slot opening in the parking brake lever during normal operation. This arrangement facilitates disassembly of the brake assembly to remove a drum that has developed a lip or step through wear in a manner which is described later.
The above arrangement secures the strut end against movement laterally to the plane of the shoe web or parking brake lever in which the slot opening is formed in both the brake on and brake off conditions. The arrangement also permits movement in the lengthwise direction of the strut, such that in the brakes on condition, when the brake shoes of the brake assembly and the parking brake lever shift radially outwardly, the or each end face of the strut and the inner end of the or each slot, can shift away from abutting engagement as required, but the channel engagement remains. Accordingly, despite any lengthwise movement of the strut, the ends of the strut remain engaged in the slots with the respective braking members. In accordance with its normal function, upon wear of the brake shoe friction lining, the adjuster strut extends lengthwise to compensate for that wear, although the mechanism by which the strut is extended can take any suitable form and is not restrictive on the present invention.
Preferably, the, or one of the end faces of the adjuster strut engages the inner end of a slot opening formed in a parking brake lever associated with a brake shoe. In such an arrangement, it is common for the body of the lever to be formed out of metal plate and to be positioned adjacent and generally in side-by-side sliding contact with the inwardly depending web of a T-shaped brake shoe with which it is associated. In that arrangement, given the close proximity of the lever to the shoe web, it is appropriate for both the parking brake lever and the brake shoe web to have slot openings for receipt of an end of the adjuster strut. In this arrangement, and as described earlier, the slots may have different elongate extents, such that the end face of the adjuster strut engages the inner end of the slot formed in the parking brake lever only.
During operation of the brake assembly to a brake on condition, the strut may undergo slight pivoting movement and therefore the slot openings in the braking members are preferably shaped to permit that pivoting movement. In one arrangement, the edges of the slot opening in the parking lever taper from the inner end to be wider at the mouth thereof, while the slot opening of the shoe web is shaped wide at the inner closed end thereof and converges toward the open mouth thereof. By this arrangement the adjuster strut may pivot as necessary without obstruction from either of the upper and lower edge portions of the slot opening in the brake shoe web, but the edge portions of the web slot opening can be arranged at the most convergent point to engage or be closely adjacent upper and lower surfaces of the respective strut end, to substantially eliminate vertical movement of that end, particularly if the end shifts in the lengthwise direction of the strut away from abutting engagement with the inner end wall of the parking brake lever to a position where the tapered edges of the slot in the parking lever are spaced further apart, so itself providing limited resistance to vertical movement.
In one preferred arrangement, the channel of the strut end portion has a width sufficient to accommodate only the edge portions of the slot opening of the parking brake lever. The side walls of the channel may extend closely adjacent the opposite sides of the edge portion to capture it in the channel against relative lateral or sideways movement. By this arrangement, the strut end section can conveniently be substantially restrained against undesirable movement both parallel and perpendicular to the plane of the shoe web.
In a second aspect of the invention, a biasing arrangement is employed for biasing the parking brake lever of a brake assembly toward the inwardly depending web of a T-shaped brake shoe. This aspect of the invention provides for engagement of an end of the adjuster strut in a slot opening of the parking brake lever and is characterised by the provision of biasing means extending between the adjuster strut and the brake shoe web which biases the parking brake lever and the brake shoe web towards each other and preferably together.
In one preferred arrangement of this second aspect of the invention, the biasing means includes a cantilever spring bias that has attachment means for attachment to the adjuster strut and that extends into engagement with a side surface of the brake shoe web to bias the web toward the parking brake lever. The attachment means may conveniently be a clip arrangement and in respect of an adjuster strut having a cylindrical body portion between respective opposite end portions, the clip arrangement may clip onto that cylindrical portion. The clip engagement may be sufficient to position the biasing means on the adjuster strut, although additional locating means such as to position the strut both angularly and longitudinally, may be provided for that purpose.
The attachment means may equally take other forms and the biasing means may for example, be attached to the adjuster strut by suitable fastening means, or such as by permanent welding attachment or riveting.
In the above arrangement the biasing means biases the brake shoe web toward and preferably into engagement with the parking brake lever in one direction only, being laterally to the elongate extent of the adjuster strut and to the plane of the web, and in that arrangement, a moment force will be generated tending to rotate the brake shoe relative to the strut. Thus, in an alternative arrangement, the biasing means imposes an equal and opposite biasing force so that no such moment force is created. In this arrangement, a pair of cantilever springs can extend into contact respectively with oppositely facing side surfaces of the brake shoe web or respective oppositely facing side surfaces of the brake shoe web and the parking brake lever. Both cantilever springs impose an equal and opposite force tending to bias the web and lever into engagement.
In a third aspect of the invention, an adjuster strut is provided with means to secure it to the braking member with which it is engaged to prevent release of the strut from its operative position. This third aspect of the present invention is characterised in that one end of the adjuster strut is formed to cooperate directly with one of the braking members so that that end of the strut is secured against release from the braking member in the assembled or operative position of the strut. In one preferred form of this aspect of the invention, the end of the adjuster strut includes abutment means suitable for engagement in the operative position of the strut to engage an abutment formed or provided on the braking member, whereby that cooperation secures the end of the adjuster strut against release from the braking member.
The abutment means can take any suitable form and in one form, it is formed by an elongate member which extends from the strut into engagement behind the braking member abutment in the operative condition of the strut. That member may for example, be of plate metal form which is fixed to the strut in any suitable manner and which has a distal end formed for positioning behind the abutment. An alternative form of this aspect of the invention includes a member that extends from the strut and engages in an opening provided in the braking member. In a preferred arrangement the opening is provided in the radially inwardly depending web of a T-shaped brake shoe and the strut member includes an abutment arranged to abut against a radially inward edge of the opening to resist release of the strut from its operative position as assembled between the braking members. The abutment may not engage the edge at all times and in a preferred arrangement, engagement only occurs when the brake assembly is in a brake on condition, or when the adjuster has failed and has stopped adjusting or extending. Thus, the opening preferably is shaped to allow movement of the abutment toward and away form the edge of the opening as the brake assembly shifts between brake on and brake off conditions.
In another form of this aspect of the invention, the end of the adjuster strut is formed to have a head and a neck portion and a substantially radial stepped surface extends at the junction between the head and the neck. In one form of this arrangement, both the head and the neck are generally cylindrical and the stepped surface is formed by an increase in the diameter from the neck to the head. The abutment formed by the braking member for cooperation with this form of strut end can be formed as part of the slot opening which is formed to receive the end of the adjuster strut. Conveniently, the abutment may be formed at the open end of the slot opening and may for example, depend downwardly from the upper edge thereof, forming a lip or catch. More than one abutment, of this kind or otherwise, may be provided.
The example forms of abutment means described above advantageously can be applied to opposite ends of the adjuster strut. Alternatively, they may be employed separately with other forms of strut ends applied to the opposite end of the strut.
The above arrangements are preferably such as to permit simple installation of the abutment means relative to the abutment to secure the adjuster strut in the place. However, in such an arrangements, it is preferable that means be provided to ensure the integrity of the arrangement during operation of the brake assembly, such as to prevent lateral disengagement of the abutment means from the shoe web abutment. Biasing means as described for the second aspect of the invention could, for example, be employed to prevent lateral disengagement of the stepped surface from the abutment, although in a preferred arrangement, the head may be bifurcated to engage opposite side surfaces of the braking member. In this arrangement, installation of the adjuster strut may be by angled insertion of the bifurcated head into the slot opening and onto the inner end thereof, followed by rotation of the strut into the general plane of the opposite brake member edge or web, whereby the abutment means, such as the stepped surface is brought into alignment behind the brake member abutment.
Each of the above described strut end forms can be employed in a single adjuster strut. That is, the strut end having an end face and a rearwardly depending channel of the first aspect of the invention can be employed in a strut in which the opposite end is formed according to the third aspect. Additionally, the method of angled insertion of the adjuster strut can be employed in a strut that has both described end forms. This is particularly the case in an adjuster strut that employs an end face of the first aspect, as that end face is not bifurcated and as such, is not required to straddle opposite sides of the braking member. As such, that end form does not have to be installed end on to the web or lever portion it is to engage, but can be rotated or swivelled into position. Moreover, biasing means as may be required, can be fitted subsequently. The third aspect of the invention is preferably applied between the end of the adjuster strut and the brake shoe web, although it could also be applied, but less preferably, to the parking brake lever.